Formation Of Undulations Research Articles

Numerical simulation of skin formation: epidermal stem cells and undulation of the basal layer

The basal layer at the base of the epidermis produces new epidermal cells once a week and plays an important role in skin formation. Interestingly, the basal layer undulates, and the amplitude of this undulation is dependent on age or disease. However, the mechanisms driving this phenomenon have not been well studied. This work simulated the skin formation process using an original method to understand the mechanism of the undulation formation in the basal layer. The basal layer has cells with both short and long cell cycles, but only the former has been examined in previous studies. Accordingly, we more accurately simulated the skin formation process by including cells with a long cell cycle and observed undulation of the basal layer. Based on our findings, we propose that the basal layer undulates due to the interaction between two types of basal layer cells.

UNDULAR BORE DEVELOPMENT OVER A LABORATORY FRINGING REEF

Several studies have reported the development of
 undular bores over fringing coral reefs (e.g, Gallagher,
 1976; Nwogu and Demirbilek, 2010) but the importance
 of this phenomenon for reef hydrodynamics has never
 been studied. Yet, the transformation of a long wave
 (e.g., swell or infragravity wave) into an undular bore
 leads to significant modifications of the wave field. The
 formation of undulations is for example associated to a
 significant increase of the leading bore height. Moreover,
 if the undulations have enough time to develop (i.e. if the
 reef flat is wide enough), the initial long wave will
 ultimately split into a series of solitons (e.g., Grue et al.,
 2008). All this is likely to affect wave run-up. As reeffronted
 coastlines are particularly vulnerable to flooding,
 a good understanding of long wave transformation over
 the reef flat, including their possible transformation into
 undular bores, is crucial.
 In this study, we investigate undular bore development
 over reef-type profiles based on a series of laboratory
 experiments. More specifically, we aim to characterize
 the conditions under which undular bores develop, and
 analyse how their development affect the hydrodynamics
 at the toe of the reef-lined beach and the resulting wave
 run-up.

Significant Volume Expansion as a Precursor to Ablation and Micropattern Formation in Phase Change Material Induced by Intense Terahertz Pulses

With rapid advances occurring in terahertz (THz) radiation generation techniques, the interaction between matter and intense THz fields has become an important research topic. Among different types of THz radiation sources, the free electron laser (FEL) is a promising experimental tool that is expected to pave the way for new forms of material processing, control of phase transitions, and serve as a test bench for extreme operating conditions in high-speed small-size electrical and magnetic devices through the exploitation of strong THz electrical and magnetic fields without the presence of interband electronic excitation. In the current work, we irradiated Ge2Sb2Te5 phase change memory material with intense THz pulse trains from an FEL and observed THz-induced surface changes due to damage as a precursor to ablation and the formation of fine surface undulations whose spatial period is comparable to or slightly smaller than the wavelength of the excitation THz pulses in the material. The formation of undulations as well as the fact that no significant thermal effect was observed below the volume expansion threshold suggests that THz-induced effects mainly contributed to the observed changes. To the best of our knowledge, this is the first experimental observation of THz-induced undulations (so-called “LIPSS”), which are of potential importance for laser material processing.

Wiggly strain localizations in peridynamic bars with non-convex potential

We discuss the static states of a peridynamic nonlinear elastic bar of finite length in a hard device, which represents a continuum description of a complex hierarchical structure with interacting long-range crosslinkers, of the type encountered in biological systems. The nonlocal character of the model requires that edge conditions are defined on a boundary layer with the same length of the horizon, affecting the solution in the bulk interior. Assuming that the constituent microscopic ligaments contain bistable units governed by a non-convex potential, we show that the development of coexisting folded-unfolded phases, either synchronized or unsynchronized, induces in the displacement field the formation of undulations at a micro-scale of the length of the horizon, associated with strain localizations triggered at the bar ends. The equilibrium paths, found numerically with a pseudo-arc-length continuation method, become unstable within a certain range of elongation, suggesting the possible occurrence of a negative-stiffness response.

Corrosion of an India produced RAFM steel in flowing lead-lithium eutectic

Reduced Activation Ferritic Martensitic (RAFM) Steel has been specifically developed to serve as a structural material for fusion reactor applications. To establish the corrosion compatibility of an India produced RAFM steel (RAFMS) with Pb-16at.% Li eutectic (Pb-16Li), an electromagnetic pump driven loop has been installed and the compatibility of this RAFMS material has been tested in liquid Pb-16Li at 773K for 5, 000h of exposure at a flow velocity of 0.6–1.5m/s. A corrosion rate of 47–50μm/y was established for the RAFM steel from weight loss measurements and the rate was found to depend on the variation in local flow velocity over the exposed samples. The corrosion attack by liquid Pb-16Li was found to be non uniform in nature which resulted in the formation of undulations over the exposed RAFMS surface. The corrosion mechanism was governed by dissolution of iron and chromium from the RAFMS matrix into the molten Pb-16Li accompanied by enrichment of tungsten near the exposed surface.

Drop formation in a magnetic fluid coating a horizontal cylinder carrying an axial electric current

We consider the effect of a magnetic field generated by a current flowing in the axial direction, in a cylindrical conducting medium on the evolution of a thin magnetic liquid film coating the outside of the cylinder. We first derive the new governing equations when a body force, due to the interaction of the magnetic dipoles in the liquid with the magnetic field due to the current in the cylinder, is included with surface tension and gravitational forces. The equations are discretized and solved numerically using an alternating direction implicit algorithm. Simulations demonstrate that the transition from a uniform coating to the formation of undulations to a final configuration of distinct drops follows a similar evolution for a wide range of cylinder radii and magnetic field strengths. Magnetic forces generally oppose the effects of gravity, and consequently the drainage toward the bottom of the cylinder is slowed, the characteristic time for drop formation is delayed, and the final coating is not confined to the cylinder bottom, but can wrap around the entire circumference of cylinder for sufficiently large magnetic fields. The range of dimensionless parameters considered are limited by the physicochemical properties of an ester ferrofluid, the maximum current density and radius of the conducting cylinder, and the susceptibility χ, where there is a linear relationship between H and M. Thus the results are applicable to actual physical situations.

Ultrastructural study of peripheral and central stroma of keratoconus cornea

PurposeAssess the lamellar organisation of the peripheral and central stroma of the keratoconus (KC) and normal cornea.MethodsFive normal and three KC corneas were fixed in 2.5% glutaraldehyde and processed for...

Cenozoic tilting history of the south slope of the Altyn Tagh as revealed by seismic profiling: Implications for the kinematics of the Altyn Tagh fault bounding the northern margin of the Tibetan Plateau

The Altyn Tagh fault (ATF) plays a significant role in the northward growth of the Tibetan Plateau, but its Cenozoic kinematics and related structural response in adjacent basins remain debated. In this study, we identified a transition zone between the ATF and the Qaidam Basin interior and termed it the Altyn Slope, based on a dense network of two- and three-dimensional seismic reflection profiles and isopach maps. Tilted by a series of E-W–trending transpressional faults that constitute the positive flower structure of the ATF, the present Altyn Slope is characterized by a southeast-dipping slope with its undulating southeastern boundary with peaks coincidentally located at the major anticlinal belts in the basin. We propose a method for restoring the Cenozoic tilting history of the Altyn Slope during different time periods by identifying growth-strata geometry from the recent isopach maps. The results show that the Altyn Slope began to form in the late Eocene (ca. 40 Ma) and continued to expand until the mid-Miocene (ca. 15 Ma) albeit with a temporally developing shape. However, the Altyn Slope shrank toward the ATF and underwent significant NE-SW–directed folding since the mid-Miocene (ca. 15 Ma), resulting in formation of undulations of its southeastern boundary. We thus infer that the left-slip motion on the ATF is divided into two distinct stages: during the first stage, ca. 40–15 Ma, the ATF was activated with slow slip rate, and most transpressional stress was converted to vertical strain, raising the Altyn Slope instead of producing strike-slip motion. During the second stage, since ca. 15 Ma, faster sinistral strike-slip motion on the ATF took place, releasing the stress beneath the Altyn Slope and inducing intense NE-SW–directed shortening within the Northern Tibetan Plateau.

A model for crack initiation in the Li-ion battery electrodes

The development of high energy density Lithium-ion batteries is of intense interest due to their application in the electric car and consumer electronics industry. The primary limiter in using high energy density battery electrodes is the cracking of the electrode material due to the severe strain caused by the charging–discharging cycles. In this paper, a linear perturbation model is used to describe the evolution of the electrode surface under stress. The driving force for the surface undulation formation is the reduction in the electrode strain energy. The kinetics of mass transport is described by the surface and volume diffusion. The model predicts that the Si electrode will develop surface undulations of the order of sub-1μm length scale on the electrode surface, showing a reasonable agreement with experimental results reported in literature. Such surface undulations roughen the anode surface and can form notches that can act as crack initiation sites. It is also shown that this model is applicable when the temperature of the system is not constant and the system is not isolated. The limitations of the model are also discussed.

Surface State Control by Ultrasonic Plastic Deformation at the Final Machining Stage

Basic results of research into the process of the surface geometry formation during ultrasonic plastic deformation of metallic materials are presented. The results of mathematical modeling of the microgeometry and surface undulation formation in the machining of materials of different hardness are also presented. Experimental validation of the adequacy of the proposed models is provided.

Large-area homogeneous periodic surface structures generated on the surface of sputtered boron carbide thin films by femtosecond laser processing

Amorphous and crystalline sputtered boron carbide thin films have a very high hardness even surpassing that of bulk crystalline boron carbide (≈41GPa). However, magnetron sputtered B-C films have high friction coefficients (C.o.F) which limit their industrial application. Nanopatterning of materials surfaces has been proposed as a solution to decrease the C.o.F. The contact area of the nanopatterned surfaces is decreased due to the nanometre size of the asperities which results in a significant reduction of adhesion and friction. In the present work, the surface of amorphous and polycrystalline B-C thin films deposited by magnetron sputtering was nanopatterned using infrared femtosecond laser radiation. Successive parallel laser tracks 10μm apart were overlapped in order to obtain a processed area of about 3mm2.Sinusoidal-like undulations with the same spatial period as the laser tracks were formed on the surface of the amorphous boron carbide films after laser processing. The undulations amplitude increases with increasing laser fluence. The formation of undulations with a 10μm period was also observed on the surface of the crystalline boron carbide film processed with a pulse energy of 72μJ. The amplitude of the undulations is about 10 times higher than in the amorphous films processed at the same pulse energy due to the higher roughness of the films and consequent increase in laser radiation absorption.LIPSS formation on the surface of the films was achieved for the three B-C films under study. However, LIPSS are formed under different circumstances. Processing of the amorphous films at low fluence (72μJ) results in LIPSS formation only on localized spots on the film surface. LIPSS formation was also observed on the top of the undulations formed after laser processing with 78μJ of the amorphous film deposited at 800°C. Finally, large-area homogeneous LIPSS coverage of the boron carbide crystalline films surface was achieved within a large range of laser fluences although holes are also formed at higher laser fluences.

Effects of impurities on silver–(ZrO2)0.92(Y2O3)0.08 interface morphology in solid oxide cells

Material interactions at elevated temperatures are major contributors to long-term degradation in solid oxide cells (SOC). The use of silver in SOC electrodes is promising due to its good catalytic activity at reduced operating temperatures and chemical stability with other SOC materials. However, no significant studies have been conducted on the interfaces between silver-based electrodes and common SOC electrolyte materials such as yttria-stabilized zirconia (YSZ). A symmetric cell configuration with applied voltage bias has been used to investigate interfaces between silver-based electrodes and YSZ electrolytes at 750°C. Surface porosity and undulations, as well as trace impurities were observed on the YSZ surfaces using electron microscopy and energy dispersive X-ray spectroscopy. The applied voltage bias enhances the migration of impurities from the electrodes to the silver–YSZ interfaces. The impurities affect the YSZ surface energy characteristics, leading to YSZ surface diffusion and undulation formation.

Prefracture Instabilities Govern Generation of Self-Affine Surfaces in Tearing of Soft Viscoelastic Elastomeric Sheets

We present an experimental study on fracture behavior of soft viscoelastic network solid films of polydimethylsiloxane (PDMS) and morphology of the fracture-generated surfaces. The most interesting behavior is the generation of fractal patterns when the storage and loss moduli are comparable. We find the formation of isolated defects and cracks ahead of the fracture-front. The observed morphology of the torn surfaces is correlated to the prefracture behavior by studying the crack propagation and coalescence. Tearing of PDMS containing 2% cross-linking agent with a terminal elastic modulus, G′, of 10 kPa, shows distinct crack patterns at different tearing speeds. Slower tearing leads to the formation of dendritic patterns of propagating fingers, whereas at the higher deformation-rates, more frequent nucleation of bubble like defects occurs. The propagating fingers develop undulations along its sides. The effects of tearing speed on the length scale of the undulations and the bubbles are studied. The formation of undulations can be explained as a result of Saffman–Taylor instability with a modified effective surface tension which accounts for the dissipative energy loss. We also study the rate of formation of the defects, which is shown to be an activated process in that they form beyond a threshold value of the local stress.

Modeling the formation of undulations of the coastline: The role of tides

An idealized model is developed and analyzed to investigate the relevance of tidal motion for the emergence of undulations of a sandy coastline. The model describes feedbacks between tidal and steady flow on the inner shelf, sand transport in the nearshore zone and an irregular coastline. It is demonstrated that an initially straight coastline can become unstable with respect to perturbations with a rhythmic structure in the alongshore direction. The mechanism causing the growth of these perturbations is explained in terms of vorticity concepts. The relative importance of tide-related and wave-driven sediment fluxes in generating undulations of the coastline is investigated for the Dutch coast. Using parameter values that are appropriate for the Dutch coast it is found that tides can render a straight coastline unstable. The model predicts a fastest growing mode (FGM) with a wavelength that is in the order of the observed length of barrier islands. The mode grows on a time scale of 50 yr and it migrates 200 m per year. The wavelength of the FGM decreases with increasing amplitude of the tidal currents. This result is consistent with data of tides, waves and the lengths of barrier islands that are located along the Dutch and German Wadden coast.

Stress-guided self-assembly in Dutcher films

Dutcher films consist of a layer of liquid sandwiched between two solid capping layers and can spontaneously self-assemble to form corrugated surfaces. The interplay between the attractive van der Waals forces across the film, and the elastic forces due to the deformation of the capping layers, produces well-defined periodic undulations. We show how computer simulations can capture both the formation of undulations in Dutcher films and the correct periodicity. Furthermore, we simulate Dutcher films which are either compressed or stretched, resulting in the promotion or suppression of undulation growth. In this manner, applied deformations can be shown to guide the self-assembly process in Dutcher films and result in the formation of highly oriented surface corrugations over large distances.

Evolution of surface waviness in thin films via volume and surface diffusion

Deformation mechanisms involving mass transport by stress driven diffusion influence a large number of technological problems. We study the formation of undulations on surfaces of stressed films at high temperature by exploring the deformation kinetics governed by volume and surface diffusion. A governing equation is derived that gives the amplitude change of such surfaces as a function of time. A parametric study is then carried out using a range of practically important input values of the film material properties. The results show that at the dominant instability wavelength, under high average stresses (giga pascal range), only surface diffusion contributes to film surface morphology evolution whereas under low stress and high-temperature conditions, both surface diffusion and volume diffusion contribute to film surface morphology evolution. Furthermore, the contribution of volume diffusion depends on the sign of the film stress, with compressive stress promoting surface roughening and tensile stress promoting surface smoothing.

Relaxation Processes of AlGaN/GaN Heterostructures Grown onto Single Crystal GaN(0001) Substrates

We study the growth and relaxation mechanisms of AlxGa1—xN/GaN heterostructures deposited by molecular beam epitaxy (MBE) onto GaN single crystals. We use a combination of transmission electron microscopy and finite element calculations (3D FE) to analyze the structure, the defects and the strained state. We compare heterostructures with 10%, 20% and 30% of Al (xAl = 0.1, 0.2, 0.3) corresponding to low misfits of 0.25%, 0.5% and 0.75%, respectively. This provides us with specimens that are dislocation free or dislocated above a certain critical thickness, to study the interplay between elastic (relaxation in laterally limited structures such as sinusoidal undulations and islands) and plastic strain relaxation. The heterostructures with Al contents xAl = 0.2 and 0.3 become unstable against undulation formation. The undulation amplitude A increases with increasing layer thickness by conserving the wavelength λ of roughly 200 nm. At an aspect ratio A/ λ ≈︂ 1/20 plastic relaxation by dislocation formation at the undulation valleys sets in. 3D FE calaculations yield a maximum elastic strain relaxation of ≈︂15%. The inhomogeneous strain distribution in the undulated AlGaN layer induces an inhomogeneous strain distribution in the thin GaN interlayer also. Detailed strain analysis yields a rather unusual result for the GaN interlayers: the valleys and the ridges exhibit almost zero strain whereas the slopes of the undulation are in compression. This result can possibly be utilized in novel quantum structures.

New strain-relieving microstructure in pure-Ge/Si short-period superlattices

High-temperature formation of highly strained pure-Ge/Si short-period superlattices (SPSs) and their photoluminescence (PL) properties are presented. We grew 99 period SPSs consisting of two-dimensional Ge layers separated by thin Si barriers. The evolution of undulations is newly observed for samples with small Si barrier thickness, which is attributed to a strain-related growth phenomenon, and similarities to the well-established island formation are discussed. Clear band-edge PL is observed in these high-temperature grown SPSs, and interesting PL properties associated with the undulation formation are presented.

Strain relaxation and thermal stability of the 3C-SiC(001)/Si(001) interface: A molecular dynamics study

Molecular dynamics (MD) and high-resolution electron microscopy (HREM) imaging are used to investigate the mechanism of strain relaxation of a model 3C-SiC(001)/Si(001) interface. It is found that the essential atomic mechanism governing this process is the formation of undulations in planes parallel and perpendicular to the interface. The net effect is the generation of misfit-accommodating dislocations, of the <110> type, which allow for structure relaxation at 2, 700 and 1000 K. MD configurations are then used for HREM image simulations. Comparisons with actual HREM images of the interface support the model interface and relaxation mechanisms proposed herein.

Silicon/silicon-germanium multiple quantum wells grown by gas-source molecular beam epitaxy: hydrogen coverage and interfacial abruptness

We have grown silicon-germanium/silicon ( Si 1 − x Ge x Si,x < 0.30 ) multiple quantum wells (MQWs) by gas-source molecular beam epitaxy (GSMBE) using disilane (Si 2H 6) and germane (GeH 4) as source gases, and have characterized their structural properties by secondary ion mass spectroscopy (SIMS), X-ray diffraction (XRD) rocking curve and transmission electron microscopy (TEM) techniques. A substrate temperature of 520°C was maintained during growth resulting in a Si and SiGe growth rate-limited primarily by hydrogen desorption kinetics. Under these conditions, surface hydrogen is expected to function as a surfactant thereby enhancing interfacial abruptness at the Si SiGe interface through suppression of Ge surface segregation. Independent of Ge composition in the Si 1 − x Ge x wells, we find abrupt interfaces, as determined from XRD measurements, and sharp SIMS decay lengths of the order of 2.5 nm/decade. For nominally identical Si barriers in all samples examined, we find thicker barriers for the structures with higher Ge content in the well. For the specimens with x = 0.30 in the wells, we find a noticeable well plus barrier period variation of approximately 5%–10% as determined from XRD rocking curves, as well as TEM evidence for the onset of strain relaxation via interface undulation formation in the first quantum well of the structure. A discussion of these results in terms of hydrogen desorption kinetics is presented.